Posted
by
CowboyNealon Friday May 14, 2004 @03:18AM
from the it-is-indeed-a-test-for-something dept.

Sam Sachdev writes "David Deutsch, a physcicist at Oxford, has designed a home test for parallel universes. Using a pin, a red laser pointer, a piece of paper, and a relatively dark room, he claims that the results from this experiment confirm the existence of parallel universes." Okay, so it may not really be proof of parallel universes, but it's a fun trick to try with a laser pointer nonetheless.

I did the experiment and have confirmed the results! Parallel universes exist, and I think David Deutsch is a genius! I have registered www.daviddeutsch.com and I will be building a shrine site to David Deutsch this week, so check back often! If there are people in this parallel universe, we need to contact them!!! Does anyone know if the people you talk to on an Ouija board are from this parallel universe?

this is what is causing the interference, "[W]hen a photon passes through one of four slits, some shadow photons pass through the other three slits." The shadow protons, then, are blocking the tangible protons, causing only three shadow slits.

These shadow protons form a parallel universe.

I'm reminded of a Star Trek: TNG episode where Data went down to some planet to collect radioactive rocks. Somehow he short circuited (or whatever - I'm not much of a Star Trek fan) and "forgot" who he was and ended up in this small village full of people that were several centuries behind the human race.

While in this village, Data sat at a table listening to a teacher explain to her student what the various forms of matter were. In one of her explanations (and you star trek geeks will have to forgive me if I'm getting the details wrong here) but she said that fire was "inside" of wood and that it could only be released by heating it up... Data interjected and said that he felt like her conclusion had to be wrong for such and such reason. And throughout the episode he demonstrates a couple of other (obvious to us) things that these unevolved people are confused about.

My point is - this guy's explanation sounds like a conclusion drawn from a limited understanding of how things really are. But IANAQP (I am not a quantum physicist)... so what do I know? Maybe it does make sense... but Parallel Universes? I don't know.. sounds like he's reaching for an answer to explain the unobservable. Given time, this ?theory? of his will be proven wrong. You know how it is... the world is flat, the sound barrier can never be broken, 640k is enough for anything, etc...

It should be added that most physicists disagree with Deutsch's conclusion that what is detected in this experiment is another universe. For brevity's sake, the argument against can be summarized as, there is something interfering with the light in this experiment, why does it have to be a parallel universe? Why can't it be just be left to something that we don't yet understand?

IANAQP either, but to the best of my understanding, the notion of parallel universes is actually a fairly logical result of empirical quantum physics experiments.

Experiments have shown that subatomic particles act very funny when you try to describe or figure them out. Basically, these particles act like particles half the time, and like waves the other half of the time, but never both at the same time. Certain well known experiments (like the banding described in the article, which are due to wave interference of light particles) have shown that particles can somehow seem to act is if they are in multiple places at once, yet they cannot be observed in multiple places at once. This has led a lot of physicists to surmise that there are 'multiple parallel universes' where that exist simultaneously. The rationale is that since the inherent particles that make up our universe are in multiple states at the same time, these inherently MPD (multiple personality disorder) particles make up a sort of multi-verse that exists at the same time in different states, thereby creating different realities/parallel universes.

...and I will reiterate, IANAQP, but it seems to me that there is a lot of going from A, B, C to X, Y, Z with nothing in the middle with that notion. We cant observe the quantum weirdness at our human-sized perceivable universe, and to assume that this quantum weirdness can cause other realities where GWBush is in Mensa seems to be a far step of logic.

If there are any quantum experts out there, and see a problem with my reasoning, or just want to educate the ignorant masses (please leave out the math, its just boring), I urge you to help.

IAAQP, and while I've not read this man's book, I'd be skeptical. Most physicists subscribe to the "Copenhagen" interpretation of quantum mechanics, which does not deal with parallel universes. The so-called "many-universe" theory has its followers but doesn't get much attention, for one reason: introducing the extra idea of multiple universes doesn't add anything to the descriptive power of the theory.

Before quantum theory was developed, most phenomena in nature were considered to be either particles or waves. This classification system broke down when particles were shown to diffract and waves were shown to be quantized. So nothing is really particle or wave, but everything has a particle or wave nature.

The canonical example is electron diffraction: shine a beam of electrons through two slits, and get an interference pattern on your photographic plate. Woo. Repeat with one electron at a time, recording each result.... and you still get an interference pattern. This presents a problem: each electron must have gone through both slits and interfered with itself. On the other hand, you can never measure the electron to be in two places at once, so we need to construct a third option. This is the idea of a superposition state: the electron is in a superposition of places (again, not actually "in" them); this superposition has wavelike properties and can interfere. When a measurement is made (by the photographic plate) the superposition "collapses" to one location.

This is where many-universe theory (to my understanding) comes in: how does the particle know which state to collapse to? Copenhagen Qm says it's random, but weighted by the superposition; i.e. quantum mechanics predicts probabilities only. Many-universe theory says that when that collapse occurs, the universe splits into a bunch of new ones, one for each outcome of the measurement. I've not yet read a good explanation (anyone have a link to one? I'd love to) of why measurement (that is, a phase-randomizing interaction with a larger system) should create a new universe.

Anyway, I hope this helps! If you are curious about QM, there's an inexpensive book by Isham that has really wonderful discussion (and even mentions many-universe). Feynman's book "QED" takes his path-integral approach and is a great layman's introduction; just don't try solving any problems with that method later (the math is rough).

The Copenhagen theory is basically 'we only know what we know' which is a cop-out if I ever heard one.

There are other interpetations besides Many Worlds, like transactional, which basically says that QM is really non-local. It has invisible waves propagating backwards in time from everywhere to everywhere, and photons and other waves happen when there's a forward wave in the opposite direction on top of invisble backwards waves.

I.e., for the slit experiment, it's not photons interfering with photons in other universes, like Many Worlds, or 'probablity waves' interfering with other waves, it's the advanced waves that interfere with each other (While, of course, going backwards in time), and to have a photon, it has to be following one of these backwards waves.

And anyone who knows anything about electromagnetic radiation is nodding their head at this point, because they already learned about these hypothetical 'advanced waves' when learning Maxwell's equations, which do not take QP into account. The transactional interpetation just says that even non-existent waves have advanced waves going the other way, and it's those waves that are interfering with each other.

"We only know what we know" is referred to as the agnostic interpretation; not Copenhagen. For a good discussion on that, check Griffith's Introduction to Quantum Mechanics. The constraints that Copenhagen interpretation places on our knowledge of nature aren't a copout; on the contrary, they make an important statement about the universe. Heisenberg's Uncertainty principle isn't just saying that we can't measure position and momentum simultaneously, but rather that an object doesn't have a well-defined position and momentum simultaneously. This theory is quite self-consistent and explains experimental data very well. And, as I'm sure you know from your electromagnetism reference, this uncertainty principle has been around for waves (with Fourier relations between position and momentum space) long before QM.

You seem to know much more about Many Worlds than I do; can you tell me how this theory (or translational) has greater predictive power than the Copenhagen interpretation? It's difficult to create a consistent interpretation of QM that both Occam and Bell would agree on.;-)

You fire a couple hundred billion volkswagons out of a *very* high speed cannon at a target a couple million miles away. In the middle of this target is a couple of slits just about the width of three volkswagons.

A couple million miles on the other side of that target, you have this larger target; we'll call it the "screen" for convenience. Now, because some of those volkswagons don't impact the slit directly, you have a "spray" of volkswagons erupting from those slits - they don't just stream thru nice and orderly, some get tangential velocities from "interfering" with one another, and with the borders of the slits.

If you map the patterns of the volkswagon impacts onto the "screen" you'll notice that they have a mathematical distribution. We call this distribution an "interference pattern". This pattern has a distinctive distribution; let's call it the "volkswagon" distribution.

So after repeated experiments, you determine that the volkswagon impacts have a certain mathematical distribution; but also you find that there is a small amount of randomness to that distribution. We'll call that randomness "quantum volkswagon mechanics" - thinking that perhaps there is some small variation in the mass, velocities, and impact geometry of each volkswagon that we can't quite qualify in our experiment. After enough experiments, we determine that our error levels are follow a distribution that has some mathematical relations to the size, mass, and average spin distrubtion of the volkswagons we fired. There may or may not be an additional statistcal factor relating to our observations, which we will call Force "X".

On down the line, we find that some of those volkswagon may shed pieces of themselves, which may or may not contribute to Force "X".

Over years of experimentation, we qualify some of those pieces, and their effects, but we know that we can't adequately predict nor determine the quantity nor various qualitative aspects of those pieces. So we develop more theories, and essentially, that's where we are at now. We suspect that there is a relation between the Q&Q+Unknown of those pieces, the volkswagons, the slits, and the fact that to detect those pieces, we have to employ smaller and smaller BBs to bounce off of them, but the more we observe, the more complex it gets.

Meanwhile we do have some math to describe the whole thing - it's called wave mechanics - but frustratingly, we can't seem to relate that math to simple things like the Apple falling on Newtons' head.

This results in thousands of journal articles by the more learned members of our society; and ultimately, after being filtered thru many learned and not-so-learned members of our society, results in a description on a information site called slashdot, in which the members debate it, including speculations on supernatural dieties, callings upon fantasm including time travel, and eruditions meant to inspire humorous responses.

Actually, the many worlds theory doesn't really say that the universe splits every time there is a quantum decision - that's a very common misinterpretation of the theory, though.

Instead, what happens is that (using Shroedinger's cat as an example) , the atom in the box is a superposition of a decayed and non-decayed quantum state. The cat, having it's life associated with said waveform become a superposition of live and dead. When you open the box, the photons (exhibiting superimposed quantum states) are read by your eyes and reported to your brain which then also splits into a superposition of states, one seeing a live cat, the other seeing a dead cat. Each quantum state of your mind then basically sees either a live or dead cat. You think that you are seeing either a live or dead cat and then assume that the cat's quantum state must have collapsed into one of those states when you looked at it.

The many worlds theory simply states that this is an illusion, you yourself have been split into multiple states and each state coexists without the knowledge of the other states and interprets the world as if waveforms collapse. Instead, the reality is that every possible quantum event not only does happen but happens simultaneously and in the same place. We just can't 'see' it.

The many worlds theory assumes that the waveforms never collapse which is basically the most faithful interpretation of the underlying equations. Any theory that talks about waveform collapse is basically tacking on extra baggage to try to explain this 'collapse' that the many worlds theory simply does away with.

Personally, I think the many worlds theory is by far the most elegant and likely explanation, all of the other versions look a lot like epicycles to me. OTOH, the many worlds hypothesis is fundamentally impossible to prove or disprove since no experiment can demonstrate that multiple quantum states coexist. (at least to my knowledge) However, lack of provability or predictive power probably says more about inherent limitations on experimental science than the validity of this theory.

Incidentally, if you examine cosmology with the many worlds interpretation, you can start applying thermodynamics laws to the universe since pretyty much every possible set of events is going on simultaneously - including events that would mimic a Big Bang.

Isn't this the same old double-slit experiment, just slightly modified? Perhaps this is new to some people, but anyone who's had the slightest interest in quantum mechanics or parallel universes should have heard of this by now.

I'll save everyone the bother of having to trudge through this whole depressing article:

It should be added that most physicists disagree with Deutsch's conclusion that what is detected in this experiment is another universe. For brevity's sake, the argument against can be summarized as, there is something interfering with the light in this experiment, why does it have to be a parallel universe? Why can't it be just be left to something that we don't yet understand?

In other words, they're using the term "parallel universe" to get people to read this. They found a neat effect with photons, yes. Might as well just call it a Terroristic Particle Exploitation, and then maybe the real media will examine it at that point. Nothing to see here, move along.

I'm not a quantum physicist, but I think I have a idea what this is about; the light waves just interfere differently with four slits. Since this Deutsch guy draws wildly different conclusions about the result, I guess he's either much stupider or much smarter than me. And since he's the university physicist and not me, I feel bad for him if it's the former.

David Deutsch is a really bright guy, but he has a problem understanding how other people think, including lots of other really smart physicists.

He believes the many-worlds interpretation of quantum mechanics adamantly, and thinks that any other interpretation is, if not outright wrong, not a useful frame of mind to understand QM.

I am also a many-worlds person, as are many other physicsts I know, but I also know many very smart quantum physicicst who are not, and I am not willing to say they are wrong (yet).

I think a historical analogy might be appropriate: Back in the day. there was substantial scientific contention over whether the sun revolved around the earth or vice versa (I am not considering the religious contention -- for a while the scientific evidence was not sufficiently clear). You see, you could reproduce all the observable motion of the planets in the geo-centric model using finer and finer epicycles. So, planets would revolve the earth, and had wobbles in their orbits that faithfully represented their entire movement patterns. Or, you could adopt a helio-centric model, in which all the retrograde motion and other strange behavior cleanly fell out of the equations. You could do the math either way, but in retrospect, the helio-centric model is a much better "interpretation" of the data than the geo-centric model, because it is useful for figuring out all sorts of other things, like gravity and conservation of momentum.

Deutcsh feels similarly about the many-worlds interpretation. But as I said, among quantum physicsts you will find the whole range of people with different levels of commitment to different theories. or interpretations.

Just to give perspective, I am a physicist who thinks that the Many Worlds interpretation (along with other things like the anthropic principle) is not only incorrect but is bad science.

It fills in a small interpretational gap but creates much larger (unanswered) questions and confusion.

If you think universe splitting occurs whenever a measurement is made, then I believe that you have a very poor understanding of what measurement is. First of all collapse is not some special/magical process and secondly you can't arbitrarily seperate the universe into observed and observer. If you are going to have splitting, it's got to happen always/everywhere and with every state basis. I would say that a preferred splitting is far more egocentric than only wanting to have 1 universe.

And assuming there is no unsplitting/suicide (and maybe even if there is) then there will universes with no measurable physics, or even worse - measurements that give a false physics. And there is no reason for us to not be in one of those universes, other than probability. Of course this could (improbable) happen here, but the point is that according to MWI it does happen somewhere. Infact, Everett proved that the crazy universes will have zero norm on the Hilbert space only if infinite measurements are taken.

I believe that you have a very poor understanding of what measurement is

If you're a physicist, perhaps you can answer a question that has been puzzling me for some time when reading about the slit experiment: what exactly is 'a measurement'? Is there a scientific definition? For example, if something detects the photon, but then discards the information does it still count as a measurement (and affect the intereference pattern)?

Honestly I don't think I could rigorously define measurement off of the top of my head, but I can very easily answer your question.

Measurement is a physical process. Despite what many people have said in the past it, has nothing to do with what you are aware of. If you detected a photon, then you did something physical to collapse the wave function. So it doesn't matter if you throw the data away or not, the physical interaction still occured.

So in other words, if you are familiar with Shroedinger's cat paradox: The cat in the box will start to rot and stink or it won't. (given food, air, water...) It doesn't matter if you don't check the cat, because the physical process between the macroscopic measuring device and the quantum state has already occured.

If I had to make a definition, I would say that measurement is a physical process between a macroscopic "classical" system and a quantum superposition of states. The interaction is in such a way that the superposition is collapsed to a single state and triggers the macroscopic measuring device into giving us a certain result - that result corresponding to what state the quantum system was collapsed to. All this with the addendum that the measuring device should be "fair" and give us a probabilities that are not modified by the measuring.

Measurement is not well understood by everyone in the community. This is probably because measurement as performed mathematically is retartedly simple. But that is a gross oversimplification of how measurement really occurs in nature. When you do the calculation for a measurement you don't even consider the interaction between the device and the system, you just assume it all works perfectly.

And even for the best among us, measurement isn't completely understood. There are serious issues with measurement dealing with time and relativity. Somebody might know these answers, but it isn't me.

Considering that this experiment was done decades ago, you'd think you could go to Radio Shack and buy a junior double-slit experiment kit for $29. I believe the experiment, because a PhD physicist co-worker of mine vouched for it, but if not for him, I'd say the whole thing is a hoax. If it was done in the 50's with primitive technology, why isn't this experiment repeated more often? Other than this laser experiment, I've only seen pictures of a wooden box w

And by old, bravehamster means OLD. Like over 200 years old. See this link [wolfram.com] for more details on Young's double slit experiment.

Basically, light behaves as a wave, and since waves can constructively and destructively interfere with one another (cast two stones simultaneously in a pond and oberve the resulting interference pattern) light will form a funny looking pattern that one would not intuitively expect on a screen some distance from the slit.

I'm no physicist (though I did get an 'A' grade for A-Level physics), but from general reading, I think that the way light behaves is pretty well understood and explained by the standard model -- in particular, by the branch of Quantum Theory known as Quantum Electrodynamics (QED). The problem isn't that we don't know what photons do, but that what they do seems so different from what our normal intuition about the world expects them to!

(A great book on QED for the interested layman is that [amazon.com] by Richard Feynman, one of the theory's originators. Not much maths, but goes into lots of detail and manages to make it fascinating.)

Remember that the 'many-worlds' interpretation of QM isn't the only one, nor even the prevailing one. Another way of understanding the Young's Slits experiment is to think of the photon interfering with all the other possible paths it could take. In reality, as far as we can tell, the photon doesn't actually take a single path anyway until it impinges on the observer -- in some sense, photons taking all possible paths half-exist before that point, so interfering with each other makes some vague sort of sense...

Anyway, the point is that the Young's Slits experiment is one of the few which are simple enough to set up in a living room, clear enough that you can see the results with the naked eye, apparently obvious in classical-physics terms, and yet (once you know that light is made of particle-like things) bizarre and inexplicable without the deep mathematics of QM.

'Fraid not, chief. Water waves are vast assemblages. Individual photons are... well... individual photons. The double slit experiment works when the photons are fired through the slits one at a time. This, if you properly grok it, is FUCKING WEIRD.

The bizarre thing in this article here is that the four-slit experiment is somehow radically new, whereas the article cheerfully (and incorrectly) explains the two-slit pattern as being commonplace.

Apparently we don't detect 'parallel universes' until we do the four-slit experiment. Read the article - this is what the author states. Now IAARP* , but I can't understand why, unless the intensities or pattern spacings do not agree with the standard 4-point interference pattern, that there is any new physics here. If we see a result from two slits, we've already shown the wave nature of light.

One of the most glaring problems with the article is where the author states

"What should happen, or is expected to happen, is that the same pattern as with the two holes appears. Light beams, according to "Fabric of Reality", normally pass through each other unaffected. So, the same pattern as the two holes, should be repeated, only brighter and slightly blurred."

If we have a pattern, we already have light beams interfering. If light beams don't interfere, we should see no pattern. This is not, and never has been, the case! The four-source pattern is a consequence of the same physics as the two-source pattern.

I'd do a nice derivation, but maths in HTML never really works.

I think that author is just deeply confused as to what is going on here. He probably hasn't read David's book; if he has, he hasn't understtod it. Now I haven't read David's book either, but I have read his papers (he and I are in the same field) and I'm sure there's nothing in the literature about the four-source pattern having any new physics not observable in the two-source pattern.

In fact, we set students multiple-source interference problems in optics in the first and second years, and no-one's noticed anything radically new happening there!

There is nothing new here. Just a redistribution of intensity to the interference maxima. Gee, if I make 100 holes, the 2-hole pattern disappears (only the central maximum remains), but I get a bunch of other dots further apart. What a big surprise!!! [*]

It's a freaking 1st year optics problem. If the book says there's some shadow interference here, the book is so wrong the author should be ashamed of being a physicist. Otherwise, the guy writing the article has unbelievably low reading abilities.

[*] for people not having taken/remembering optics from college, the interference (a.k.a. principal) maxima are suppressed by the diffraction envelope (which has its own maxima) of each slit. The more holes you add, the further apart the interference maxima move. Different numbers of slits have different interference patterns.

This is because what is reaching the last filter in the second case does not have the same polarization that it had in the first case.

After passing through the first filter the light has been polarized in some direction- all of the perpendicular components have been removed by the filter.

In the first case, there is only one other filter, oriented at 90 to the first one. This will only allow the components of light polarized 90 to the first screen's orientation to pass. But all of those components were removed, so nothing gets thru.
In the second case, your filter is not oriented at 90, but at 45 to the first filter. Hence it will only components of light at 45 to the first filter's orientation to pass. But using vector analysis, we can break that orientation up into two vector components (that match up with the orientations of the first and third screens), and see that some light will get thru. How much? Well, cosine of 45 is 1/sqrt[2], and the intensity of the transmitted light goes as the square of that, so 1/2 of the light coming from the first filter gets through the 2nd filter.

Also, 1/2 of the original light went thru the first filter- this assumes a random distribution of polarizations of incoming light, i.e. unpolarized light.

Since the third filter is oriented at 45 to the second, we get another factor of 1/2. Totaling up all 3, we get 1/8 of the original intensity. I hope this makes sense. It probably won't unless you are comfortable with vectors.

It depends on your Interpretation of the underlying quantum mechanics.

The normal double slit experiment doesn't actually tell you very much. It's when you do the double slit experiment with *single* photons that the truth becomes spooky.

The reason being that even with single photons you get the same pattern on the wall. The question is did the photon interfere with itself or was there a 'ghost' photon that went through the second hole that interfered with our photon but this ghost exists in a Parallel universe?

Well, if you read the Feynman lectures in physics he does a good thought experiment to clear this up a bit. Imagine we have a second single photon beam. The idea is that we measure the photon going through the slits to see which slit it actually goes through. At first the frequency is too high and it destroys the interference pattern.

As we turn the frequency down the pattern begins to reappear but at the precise moment that the pattern does reappear we are unable to view which slit the photon went through. The frequency of the light is too low to clearly resolve the slits and hense which slit the particle went through - they've blurred into one slit.

So the question of which intepretation is correct is more a point of philosophy. We can't decide which one is correct because quantum mechanics wont let us take a measurement.

The reason being that even with single photons you get the same pattern on the wall.

Well, That's not completely correct. You only get the same pattern on the wall if you plot the statistical distribution for the photons. Although SINGLE photon won't make an interesting interference pattern, the probability of where the photon hits follows that pattern (thus a statistical record of many photons will produce the image).

I'm just preemptively clairifying this point, since this confused me years ago when I first learned about the experiment.:-)

In my understading of the multiverse theory, there are infinitely many parallel universes. This article seems to be talking about probability, which in the face of an infinite number of chances, is moot. It's guaranteed to happen. Obviously, this isn't the case. Someone want to clear this up for me?

Let's go back to our trusty two slit experiement. We have a pattern on our wall. That pattern is a distribution of photons. The question you have to ask is: "Can we predict where a photon is going to land on our pattern?"

Well, where there is darkness, we know the probability of a photon landing there is very small. In the bright areas its a rather good probability but other than that we can't say much. We can't before the time tell which bright fringe a photon will land at.

The many worlds theory explains this by saying that there is a different universe in which the photon lands in each of the bright strips. We see it land in whichever strip because we happen to be in one of those universes

The many worlds theory explains this by saying that there is a different universe in which the photon lands in each of the bright strips. We see it land in whichever strip because we happen to be in one of those universes

Doesn't this break causality?

I read "Schrodinger's Kittens" recently (so I'm obviously an expert;) and I couldn't help but feel that photons sounded like they were riding a wave. The wave passes through both slits, but the thing we measure as a photon only goes through one. Since the w

I know jack squat about quantum mechanics but I saw a brief explanation of the double slit experiment on TV and the guy explaining that photons from a similar parallel universe were interfering with the photons in ours but there was seemingly no attempt to prove this other than to show that there was a phenomenon that we did not understand and that parallel universes was a possible explanation. I immediately thought up a method which would lend credibility to the parallel universe theory if the test succee

Well, the double slit experiment, technically, shows that light behaves as a wave. It's the double slit experiment that had people convinced for a couple hundred years that light was a wave. The weirdness comes in when you send out what we believe to be a "particle" of light, and it still behaves like a wave by demonstrating interference patterns, which implies that the "particle" has somehow travelled through both slits.

It actually gets weirder than that: If you, in any way, detect which slit the photon

I hate to break it to you, but that's not a valid interpretation; a single photon can and does interfere with itself. Where you think of a particle, QM sees that a "photon" is a localized wave packet, represented by a probability wave that has useful values in a small volume (because it still looks like a particle) but exists everywhere. This probability wave can and does go through the different holes, and what you get out is effectively an interference of the photon with itself. This is the basic idea behind Feynman's "sum of all histories" (properly, the path integral formulation of QM) approach, that looks at all possible paths - in this case, all the holes.

In english, The photon is not a point, only when it hits something does it act like a point, as it only hits one point. A photon is a weird fuzzy thing that is mostly here, and partly here and over there, but a little bit everywhere else. It interferes with itself because it squeezed through both holes, and because it squished itself through both holes, it's shape (places where it mostly and partly was) changed, and so there are some places that it is more likely to hit, and some places that it can't hit.

I'm no physicist, but the article talks about photons and their properties, then mid sentence and afterwards begins referring to them as protons and THEIR properties, then goes on with a description of some photon/proton hybrid logic

The point (briefly and ineptly mentioned in the article) is that if you _do_ have only one photon leaving at a time (such experiments have in fact been performed) you get the same diffraction pattern.

So a single photon somehow passes through all four slits at once and interferes with itself.

Unless you try to detect which slit it's going through -- then the pattern disappears.

Now all this can be explained in terms of wave functions, state superposition, and wave function collapse when a measurement occurs. But the point is, that "wave function collapse when a measurement occurs" and "parallel universe with shadow photons" are about equally bizarre phenomena. And assuming they give the same predictions for results of experiments, neither is more "correct" than the other.

Of course this article doesn't cover the question of whether the two theories give the same predictions... which is where the pseudo-scientific part comes in.

Sam Sachdev [mailto] writes "David Deutsch, a physcicist at Oxford, has designed a home test for red laser pointers. Using a pin, a parallel universe, a piece of paper, and a relatively dark room, he claims that the results from this experiment confirm the existence of laser pointers." Okay, so it may not really be proof of laser pointers, but it's a fun trick to try with a parallel universe nonetheless.

Why this is suspicious:
It seems a little strange that only _one_ source is cited throughout the article, david deutsch.
False information by third paragraph:
First, a red laser pointer is needed. I found one at Radio Shack for $19, not including the triple A batteries that were needed. The red color of the laser pointer is important. The red light, unlike the white light of a flashlight, which is a composite of many colors, doesn't fray as white light does. The red light, specifically, of the laser pointer casts more specific shadows - which is what this experiment does. A flashlight, according to Deutsch, can probably be substituted. A filter, however, is going to have to be placed over the white beam. The filter, can only be red colored glass; paper or any other filter won't work.
Yes, a laser is needed, but not because it is red, in fact any color laser should work, red is just the cheapest. The reason for a laser is that it provides coherent light, that is all the light that is emited is in phase. This is necsessary for the interference.
Sachdev tries to explain the interference soley in terms of particles, when in fact the light is behaving as a wave. He is entirely neglecting the wave-particle dualty, and resorts to parrallel universes to explain it in terms of particles.

First off, the author can't keep straight the difference between a photon (a boson) and a proton (fermion).

Second of all, he credits David Deutsch with an idea that most certainly is not his. Both the notion of wave functions (what this article is talking about) and the idea that this somehow relates to parallel universes are older than I am.

This is not a revolutionary idea, and it is not really a controversial one either, as the author of the article seems to indicate. This is just one explanation of a curious quantum mechanical effect. There are other explanations, and they all describe what happens quite accurately. They may each have their own proponents, but really none of them is wrong--they are just different interpretations.

I generally do not like griping, but this write up is positively abysmal. It is no offense to David Deutsch--I am sure he is a quite competent individual. But I do not think the author of this paper actually read his book. It sounds too much like the BS I would string together from reading the first few chapters and the epilogue when I had a book assignment in schoool.

What a bunch of unintelligible nonsense. I'm sure David Deutsch would explain this differently. Whatever he told the author of the article has been lost somewhere. Probably in the vacuous head of the author. He doesn't mention how light behaves as particles AND waves at the same time. He talks about "shadows" going dark. In fact, when I was done reading the article I wasn't sure what he meant by his use of the word "shadow" at all. The writer did a terrible job of explaining what's going on in this experiment and what it's supposed to represent.

I tried the experiment myself, and Dr. Deutsch is right! Through the holes, I saw images from many parallel universes, worlds in which Columbus discovered Europe, Lincoln shot President Booth, and Germany and Japan saved the world from Nazi America and Fascist Britain in WWII. (However, Michael Jackson is a disfigured weirdo pervert in every parallel world. Must be a fundamental physical law, like the speed of light.)

This demonstrates that light can act like a wave, and have a diffraction pattern.

The "parallel universe" part comes in to explain why it still works if you fire single photons, but since you can't fire single photons (or easily check the results if you could), this isn't really a "home test" of any use.

The fact that single photons can make a diffraction pattern, seemingly interfering with themselves, is a truly weird feature of quantum mechanics (but then, I repeat myself -- quantum mechanics is always truly weird!). And one of the explanations proposed is that light in parallel universes is somehow causing the interference with the single photons in this universe.

Another explanation is that light sometimes acts as a particle, and sometimes as a wave, and when you detect a single photon coming through a slit, you are forcing that photon to act like a particle, and it will not throw a diffraction pattern; but if there is no measurement to decide which slit the photon passed through, the light can act as a wave instead of a particle, and can have an interference pattern.

There are two sets of universes, each containing a version of our photon, one set in which the photon passes through the left slit and one set in which it passes through the right. (Actually there are an even greater number of universes in which the experiment is never carried out in the first place, but we are ignoring those.) The photons are particles that carry a property called "quantum phase" which oscillates as they travel. Two universes which are identical except for the photon arriving at a certain point on the film with opposite phases, cancel each other out. Neither one is "real". Maybe it is more correct to say that the multiverse cannot contain two such contradictory universes in the first place, rather than to imagine them existing, and then meeting and going "poof".

In the Many Worlds Interpretation of quantum mechanics, each possibility is represented is represented by a branching universe. So if you flip a coin, there is a universe in which it goes heads, and one in which it goes tales. (that is oversimplifying a bit--there would in fact be infinitely many of each)

Well, how do you know if you live in such a "multiverse"? The answer was proposed by Max Tegmark just a few years back:
Take a gun, put it to your head, and pull the trigger. Repeat several times. If the multiverse model is correct, then your "self" will continue to exist only in those universes where the gun does not fire. So if you try and pull it a bunch of times and nothing happens, you must be one of the many parallel yous who happens to live in a universe where, in spite of probability, the gun did not fire.

Of course, I would not recommend trying it. If the MWI is correct, well, then in another universe you already have tried.

This is a very old experiment, and a well-known phenomenon. It was even one of the answers on slashdot's poll for favourite physics experiment (and my personal favourite).

Even the idea that it is proof of parallel universes is not original. Michael Crichton made that claim in his book Timeline. It's an excellent book (despite the horrible movie loosely based on it), but it is fiction.

"Sam Sachdev, a graduate of the University of Iowa, is also a freelance science journalist. In addition, for between three and four hours a day, he writes fiction. Presently, he's writing a play about the relationship between gay-rights and marriage, in the U.S., and Christianity."

I would like to point to a former post I made which mentionned this earlier, about the two-slit experiment:

"An interesting theory trying to explain this seemingly inexplicable result, is by taking the hypothetical possibility that the bands are created by photons that exceed the speed of light. Only when they revert to another (visible) quantummechanical state (by hitting the wall, for instance) do they become noticable.

This is not impossible, because, contrary to what most ppl think, lightspeed is in fact an average; within one beam, there can be photons that are moving slightly slower, and photons that move slightly faster then the speed of light.

This, however, leads to the conclusion that those particular photons come from - at least potentially - another time or space. So, the film 'paycheck' might not be complete bullocks after all (though it's doubtfull we are ever going to be able to create a usefull 'time-viewing' tool out of it).

Then again, never say never, as Bill Gates with his '640K is enough for everyone' can vow.

The theory about another 'space', in contrast, leads us to the possibility that those photons actually come from parallell universes. It seems SF, but it are, in effect, valid scientific hypotheses which deserve further investigation.

After all, apart from these theories, there *is* no explication for the result of that experiment."

While I have had a lot of criticism for the 'faster then light' therory (though I didn't invent it, and it *was* proposed as a hypothesis), the 'parallel universes' hypothesis is a bit more well known, it would seem.

Firstly, as others have pointed out, this is essencially the double slit experiment. In this case, because he's just using laser light, this simply demonstrates the interference of waves.

More interesting results come from when you pass through single electrons or photons one at a time, and they show the same behaviour, but this experiment does not demonstrate this. Nor is the only explanation for this to assume parallel universes. The so-called "Many Worlds" theory.

In fact, according to this [kathryncramer.com], the Many Worlds theory has been invalidated by a recent experiment.

So not only does this laser-pen experiment not prove the existence of parallel universes, but the Many Worlds explanation of the phenomenon has been potentially been already disproven.

Another point: regarding your link to the blog, I mention the following comment from Physics Forums [physicsforums.com]: The blog did not indicate two things: (1) that there's no references to the Afshar experiment and (2) that it is having problems in the refereeing stage. In fact, unconfirmed reports have indicated that the Afshar experiment report that was uploaded to the e-print archive was removed, something that is unheard of for arXiv. Until peer review of this supposed invalidation, lack of skepticism is silly.

Oh, it gets better. Gotta love this comment [livejournal.com] for example. Using Google reveals a lot of similar responses. In other words, it's nonsense and we can all forget about it. Too bad that comment got moderated so high and now people that didn't bother to doublecheck will actually think that some QM interpretations have been invalidated.

This is just a version of the classic double-slit experiment of quantum mechanics.Deutch believes in multiple universes. He uses this belief to explain the results, but typically for Deutch he says the results prove his belief, which is nonsense. There are many other explanations and one of the strangest aspects of quantum mechanics is that there is probably no way to say which explanation is right. Some of the other explanations are equally weird: the Copenhagen interpretation says that particles only 'collapse' into definite positions when something looks at them. The Transactional interpretation (my favourite) explains the results by assuming that particles are continually interacting back and forth in time. Other ideas include the suggestion that quantum states collapse into what we see when things get large enough for gravity to be significant (to put it simply).

Of course, the most sensible interpretation is to take the scientifically humble attitude and say that we don't fully understand what is going on and can't explain it, rather than to arrogantly assume all results 'prove' your personal metaphysical beliefs.

Ok, this proves quantum theory. Saying that parralel universes therefore follow is either gross oversimplification or just forcing your metaphysical opinions of a physical theory on others.
And yes, Young's double slits, old news.
Also, this doesn't show that light is a wave. This demonstrates that a photon can be placed in superposition. This experiment has also been done with electron beams, whole atoms, and (IIRC) C60 (buckminsterfullerene), and they make interference paterns. Now atoms are definitely particles.

Dunno if anyone mentioned it, but Michael Crichton's Timeline was based on time travel using the concept of parallel universes. Crichton neatly details an experiment to show the principle of entanglement. (sad that the movie did not deal with the science at all) Read the book for some nice fun with this concept.

A photon isn't a particle, it isn't a wave. It's a photon. So many people don't understand that.

So when a bunch of photons show up as an interference pattern, they think of it as waves and the model produces accurate and useful results. When people knock electrons off atoms with gamma rays, they think of the photons as particles and the model produces accurate and useful results. When the two models come together, people have the hardest time understanding it because they forgot the most elementary rule of models:

ALL MODELS ARE WRONG.

As I understand it, under the standard model, we figure out if a photon interacts with another particle by integrating over the set of all possible paths the photon can take in the meantime, producing a probability. That seems like a pretty sound model to me. Does this model require more than one photon to explain diffraction? Nope. Does it talk about waves and interference? Nope. It doesn't mention parallel universes either. This is the model that scientists started using when they realized that both the particle and the wave models were not only wrong, but they didn't always produce useful results.

The problem I have with the claim that this is proof of parallel universes is that parallel universes doesn't add anything to the existing theory. Now, if the parallel universe theory were to predict something disagreeing with the standard model, anything at all whatsoever, it would be useful. However, as it stands, I see the theory as just a more complicated explanation of the standard model. It may be true, but it doesn't seem useful, and usefulness is the only desirable trait in a model.

No scientist understands the laws of the universe, scientists don't even agree on the laws. They don't agree on string theory, on the existance of black holes, on the fate of the universe, on the presense of dark matter, or interpretation of quantum mechanics. If anyone were to ask me about any of those, I would give a resounding maybe. Heck, there are scientists proposing revisions of Newton's law F = m*a to explain discrepancies in galactic rotation.

I just get sick of scientists peddling useless but imaginative models to the public like this. That's what philosophers / fiction writers are for.

The experiment sounds interesting, but I would never, never, never read anything this guy writes. It sounds like he has a very simple experiment that could be explained in about a paragraph and we had to make it hundreds of words. There is so much filler and useless extra language that I wanted to scream.
Take a piece of paper with 2 holes in it. Shine a laser pointer thru it. Look at the wall. Put two more holes in it. Look at the wall again. Now I will explain the phenomenon....
There, I just rewrote his whole article.
Argh!!!!!!

You have understood nothing. The phenomenon is real and one of the strangest and most spooky things in physics. It shows that it it possible to get a particle (in this case a photon) to interfere with itself.

The only question is how you interpret it. The first interpretation, created by Einstein, Bohr and other dignitaries of the time, was the "Copenhagen Interpretation" which requires an "observer".

The "Many-worlds interpretation", first thought of in the late fifties gets rid of the need for a mystical observer by introducing parallell universes, where entangled particles can still interfere with each other.

This interpretation is championed by many of the leading physicists. For example Deutsch and Murray Gell-Mann.

I believe Feynman has a strange third interpretation involving particles travelling backwards in time, that cancel out the waves of forward travelling particles at specific points in space-time.

You have understood nothing. The phenomenon is real and one of the strangest and most spooky things in physics. It shows that it it possible to get a particle (in this case a photon) to interfere with itself.

and so, out of guilt and self-loathing, it hides itself from the observer?

There isn't really anything mystical with the "observer", after all, it's only a way of talking about an interfering particle, isn't it?As I understood it, superposition of states is the way it's ususally seen (and described as), but some physicists want to keep things more deterministic, and introduce parallel (deterministic) universes instead of a single indeterministic one.

But the observer is a passive receptor (in our case). I entirely sympathise with the objection - why is a human's retina the defining absorber, whereas those photons that strike other absorbing surfaces do not collapse the wave function.

As it happens I am a super-Copenhagen believer, that is, our function, as conscious entities, is to observe the many possible universes and 'select' the real one.

A human retian is not the defining absorber. Interaction with any macroscopic system collapses the wavefunction. This is why if you run the two-slit experiment but put a detector by each slit to watch for which slit the photon passes through, you don't get an interference pattern. The interference from the macroscopic detector at the slits collapses the wavefunction. Consciousness does not enter into it, that's just pseudo-mystical nonsense.

Quantum Physics is the single most successful theory in the history of science.

The interpretation of Quantum Physics is the single least scientific endeavour known to man.

It was fine for great physicists to propose these interpretations, but for anyone to accept them as "real", or to say one interpretation is more "correct" than another, is wrong-headed. What gets me is the people who then springboard off their favourite interpretation to make wild sweeping extrapolations with no scientific backing whatsoever.

(F = ma ran into a couple of problems a while back. Some guy named Albert studied the results of some experiments by some other guys named Michaelson and Morley, and decided that at high speeds, the concepts "m" and "a" started to get a little freaky. "d" and "t" were found to be pretty dicey as well.)

The parent post is right: quantum mechanics is one of the most successful theories in the history of science. By successful, I mean that it (a) accurately predicted measurements that were not explainable by previous theories, (b) has not predicted any results that are demonstrably incorrect, and (c) did all of this with a fairly simple (minimal) formulation.

Those three statements can be made about any solid theory, but QM has one unique characteristic. Unlike (say) Mendellian genetics, which challenges us with the difficult (but tractable) problem of "How did a biochemical mechanism for inheritence of traits ever come to be?", QM challenges us with "Why does the universe behave in a way which is contrary to our fundamental sense of reality?" This is not a knock against the theory, though. It just raises the deeper question: "Why should we assume that *our* fundamental sense of reality f-ing matters?" Despite almost a century of incomplete attempts to understand what quantum mechanics "means", the theory itself keeps on keepin' on - unfailingly accurate in its predictions, blithely indifferent to its metaphysical ramifications.

A different post in this thread makes the key point for grasping the various interpretations of QM: they are just *interpretations*. They have no bearing on what is "real" or "not real". All that is real (AFAWCT) is that the predictions of QM are accurate. Whether that means phantom universes, wave-particle duality, or little green men, is really of no importance until one of those interpretations leads to novel, verifiable predictions.

The article was not only an atrocious and pompous bit of writing, it was bad reporting. To represent this scientist's thesis as "novel" or even "scientific" just shows that the author doesn't know beans about the history of quantum physics.

Disclaimer: IWOAPUTDCB (I was only a physicist until the Dot-com boom).

Good to see people describing the quantum phenomenons as 'spooky'. Really. People tend to believe that everything's solved in physics. One has to keep in mind that physics only build *models of reality*.Of course, in daily physics speak, one talks about 'the electrons that hit the surface' etc. because there is a underlying theory which describes most of the experiments with sufficient precision. Daily physics is simply more like engineering than thinking about the world itself.But electron's are only human-invented concepts. Very successfull concepts, indeed. But only concepts. Maybe they're 'really resonances of some weird field' yet to be discovered. But what are resonances and 'this weird field'? They're also invented concepts. Concepts to aid 'understanding'.Many of my fellows (I'm studying physics) just believe they're electrons which properties and formulas to describe them. I don't. I take them as always incomplete, yet successful and helpful models of reality. Maybe this is just an arrogant statement and my 'open-mindedness' now brands me a crackpot to be modded down.But I am no crackpot. I don't believe in UFOs and stuff.

Regarding the 'multiverses': IMHO, one very important question remains: How you as yourself evolve in this multiverse. What decides which part you take in the multiverse? Why is it that you only see one universe, that you only exist in one universe? What decices where you/your conscience goes? Maybe this is the free will? I don't know but this bothers me.

The computer in front of you is also nothing but a concept. It's just your bain's interpretation of the data coming through your senses. It's part of your brain's model of reality. Yet you'd surely call the computer real. Why? Because your brain's model works for it. The computer behaves as if it were really such a thing as your brain's model says.Now, for electrons, it's the same: In all experiments so far they behave as if they were exactly what the theory describes. And therefore they are real, in the same sense as the computer in front of you is real.

True. But the distinction I think he's trying to describe is like comparing Newtonian physics and Einsteinian physics. For most everyday stuff involving objects we can see and speeds easily measured, Newtonian physics work well (e.g. using F=ma to measure acceleration of a car). But as you approach the speed of light or supermassive objects, Newtonian physics' inaccuracies appear. The more extreme the conditions are, simpler models show their inadequacies and a more detailed accurate model is constructed. The same thing applies with electrons -- the basic model of an electron works reasonably well for things such as building simple electronic circuits and maybe particle collisions (I can't really say for sure, IANAP), but as more extreme conditions are explored, a more detailed model may be needed to explain electron behavior. Maybe it's like a fractal -- the closer you look at the edges, the more details that appear.

While Einstein certainly helped to establish quantum mechanics, he did not like the Copenhagen interpretation because he could not bring himself to accept a non-deterministic universe.

It's rather interesting that after his work on relativity some people asked him about his religious beliefs to which he replied that they did not matter, but ultimately they did matter. Einstein later said that his religious beliefs were losely based on those of Spinoza. Basically Spinoza said that the universe is itself a pa

He is talking about the classical double slit experiment. The results of that experiment are correctly predicted by quantum physics because you need to treat photons as waves and not as particles here.

The author however wants to explain the results treating the photons as particles only. I must admit I have no idea why this leads him to the parallel universe theory.

In my opinion that theory is not needed here as we already have an excellent model (the quantum physics) that predicts those results extremely exactly. We must not forget that quantum theory (and its application in particle physics) is the most accurate theory / model in the world. No other theory other than quantum theory matches as exactly with the experimental results (up to 10 to the power of -9)!

We must not forget that quantum theory (and its application in particle physics) is the most accurate theory / model in the world.

True.

However, we don't understand how it works. Quantum theory is a bunch of constants and equations, and it all works but we don't understand why. The "many-worlds interpretation" of quantum mechanics suggests that parallel universes have something to do with how quantum mechanics works.

P.S. We also don't understand why quantum mechanics rules apply at very small scales, b